Polymerization process

ABSTRACT

The invention relates to continuous gas fluidised bed polymerisation of olefins, especially ethylene, propylene, or mixtures of these with other alpha olefins, wherein the monomer-containing recycle gas employed to fluidise the bed is cooled to condense out at least some liquid hydrocarbon. The condensed liquid, which can be a monomer or an inert liquid, is separated from the recycle gas and is fed directly to the bed to produce cooling by latent heat of evaporation. The liquid feeding to the bed can be through gas-induced atomiser nozzles (FIG. 2), or through liquid-only nozzles. The process provides substantially improved productivity of gas fluidised bed polymerisation of olefins.

This is a continuation of application Ser. No. 08/602.013, filed Feb.15, 1996, now U.S. Pat. No. 5,733,510 which is a divisional ofapplication Ser. No. 08/256,052, filed Jun. 24, 1994, and now U.S. Pat.No. 5,541,270, which is a 371 of PCT/GB94/01074 filed May 19, 1994.

BACKGROUND OF THE INVENTION

The present invention relates to a continuous process for the gas-phasepolymerisation of olefins in a fluidised bed reactor, and in particularto a process having improved levels of productivity.

Processes for the homopolymerisation and copolymerisation of olefins inthe gas phase are well known in the art. Such processes can be conductedfor example by introducing the gaseous monomer into a stirred and/orfluidised bed comprising preformed polyolefin and a catalyst for thepolymerisation.

In the fluidised bed polymerisation of olefins, the polymerisation isconducted in a fluidised bed reactor wherein a bed of polymer particlesare maintained in a fluidised state by means of an ascending gas streamcomprising the gaseous reaction monomer. The start-up of such apolymerisation generally employs a bed of preformed polymer particlessimilar to the polymer which it is desired to manufacture. During thecourse of polymerisation, fresh polymer is generated by the catalyticpolymerisation of the monomer, and polymer product is withdrawn tomaintain the bed at more or less constant volume. An industriallyfavoured process employs a fluidisation grid to distribute thefluidising gas to the bed, and to act as a support for the bed when thesupply of gas is cut off. The polymer produced is generally withdrawnfrom the reactor via a discharge conduit arranged in the lower portionof the reactor, near the fluidisation grid. The fluidised bed comprisesa bed of growing polymer particles, polymer product particles andcatalyst particles. This reaction mixture is maintained in a fluidisedcondition by the continuous upward flow from the base of the reactor ofa fluidising gas which comprises recycle gas from the top of the reactortogether with make-up feed.

The fluidising gas enters the bottom of the reactor and is passed,preferably through a fluidisation grid, to the fluidised bed.

The polymerisation of olefins is an exothermic reaction and it istherefore necessary to provide means to cool the bed to remove the heatof polymerisation. In the absence of such cooling the bed would increasein temperature until, for example, the catalyst became inactive or thebed commenced to fuse. In the fluidised bed polymerisation of olefins,the preferred method for removing the heat of polymerisation is bysupplying to the polymerisation reactor a gas, preferably the fluidisinggas, which is at a temperature lower than the desired polymerisationtemperature, passing the gas through the fluidised bed to conduct awaythe heat of polymerisation, removing the gas from the reactor andcooling it by passage through an external heat exchanger, and recyclingit to the bed. The temperature of the recycle gas can be adjusted in theheat exchanger to maintain the fluidised bed at the desiredpolymerisation temperature. In this method of polymerising alphaolefins, the recycle gas generally comprises the monomeric olefin,optionally together with, for example, diluent gas or a gaseous chaintransfer agent such as hydrogen. Thus the recycle gas serves to supplythe monomer to the bed, to fluidise the bed, and to maintain the bed atthe desired temperature. Monomers consumed by the polymerisationreaction are normally replaced by adding make up gas to the recycle gasstream.

It is well known that the production rate (i.e. the space time yield interms of weight of polymer produced per unit volume of reactor space perunit time) in commercial gas fluidised bed reactors of theafore-mentioned type is restricted by the maximum rate at which heat canbe removed from the reactor. The rate of heat removal can be increasedfor example, by increasing the velocity of the recycle gas and/orreducing the temperature of the recycle gas. However, there is a limitto the velocity of the recycle gas which can be used in commercialpractice. Beyond this limit the bed can become unstable or even lift outof the reactor in the gas stream, leading to blockage of the recycleline and damage to the recycle gas compressor or blower. There is also alimit on the extent to which the recycle gas can be cooled in practice.This is primarily determined by economic considerations, and in practiceis normally determined by the temperature of the industrial coolingwater available on site. Refrigeration can be employed if desired, butthis adds to the production costs. Thus, in commercial practice, the useof cooled recycle gas as the sole means of removing the heat ofpolymerisation from the gas fluidised bed polymerisation of olefins hasthe disadvantage of limiting the maximum production rates obtainable.

The prior art suggests a number of methods for removing heat from gasfluidised bed polymerisation processes.

GB 1415442 relates to the gas phase polymerisation of vinyl chloride ina stirred or fluidised bed reactor, the polymerisation being carried outin the presence of at least one gaseous diluent having a boiling pointbelow that of vinyl chloride. Example 1 of this reference describes thecontrol of the temperature of polymerisation by the intermittentaddition of liquid vinyl chloride to fluidised polyvinyl chloridematerial. The liquid vinyl chloride evaporated immediately in the bedresulting in the removal of the heat of polymerisation.

U.S. Pat. No. 3,625,932 describes a process for polymerisation of vinylchloride wherein beds of polyvinyl chloride particles within a multiplestage fluidised bed reactor are kept fluidised by the introduction ofgaseous vinyl chloride monomer at the bottom of the reactor. Cooling ofeach of the beds to remove heat of polymerisation generated therein isprovided by spraying liquid vinyl chloride monomer into the ascendinggas stream beneath the trays on which the beds are fluidised.

FR 2215802 relates to a spray nozzle of the non-return valve type,suitable for spraying liquids into fluidised beds, for example in thegas fluidised bed polymerisation of ethylenically unsaturated monomers.The liquid, which is used for cooling the bed, can be the monomer to bepolymerised, or if ethylene is to be polymerised, it can be a liquidsaturated hydrocarbon. The spray nozzle is described by reference to thefluidised bed polymerisation of vinyl chloride.

GB 1398965 discloses the fluidised bed polymerisation of ethylenicallyunsaturated monomers, especially vinyl chloride, wherein thermal controlof the polymerisation is effected by injecting liquid monomer into thebed using one or more spray nozzles situated at a height between 0 and75% of that of the fluidised material in the reactor.

U.S. Pat. No. 4,390,669 relates to homo- or copolymerisation of olefinsby a multi-step gas phase process which can be carried out in stirredbed reactors, fluidised bed reactors, stirred fluidised bed reactors ortubular reactors. In this process polymer obtained from a firstpolymerisation zone is suspended in an intermediate zone in an easilyvolatile liquid hydrocarbon, and the suspension so obtained is fed to asecond polymerisation zone where the liquid hydrocarbon evaporates. InExamples 1 to 5, gas from the second polymerisation zone is conveyedthrough a cooler (heat exchanger) wherein some of the liquid hydrocarboncondenses (with comonomer if this is employed). The volatile liquidcondensate is partly sent in the liquid state to the polymerisationvessel where it is vaporised for utilisation in removing the heat ofpolymerisation by its latent heat of evaporation. This reference doesnot state specifically how the liquid is introduced into thepolymerisation.

EP 89691 relates to a process for increasing the space time yield incontinuous gas fluidised bed processes for the polymerisation of fluidmonomers, the process comprising cooling part or all of the unreactedfluids to form a two phase mixture of gas and entrained liquid below thedew point and reintroducing said two phase mixture into the reactor.This technique is referred to as operation in the "condensing mode". Thespecification of EP89691 states that a primary limitation on the extentto which the recycle gas stream can be cooled below the dew point is inthe requirement that gas-to-liquid be maintained at a level sufficientto keep the liquid phase of the two phase fluid mixture in an entrainedor suspended condition until the liquid is vaporised, and further statesthat the quantity of liquid in the gas phase should not exceed about 20weight percent, and preferably should not exceed about 10 weightpercent, provided always that the velocity of the two phase recyclestream is high enough to keep the liquid phase in suspension in the gasand to support the fluidised bed within the reactor. EP 89691 furtherdiscloses that it is possible to form a two-phase fluid stream withinthe reactor at the point of injection by separately injecting gas andliquid under conditions which will produce a two phase stream, but thatthere is little advantage seen in operating in this fashion due to theadded and unnecessary burden and cost of separating the gas and liquidphases after cooling.

EP173261 relates in particular to improvements in the distribution offluid introduced into fluidised bed reactors and refers in particular tooperation in the condensing mode as described in EP89691 (supra). Moreparticularly, EP173261 states that operation using an inlet to the baseof the reactor (beneath the distribution plate or grid) of thestandpipe/conical cap type (as depicted in the drawings of EP 89691) isnot satisfactory for a condensing mode of operation due to liquidflooding or frothing in the bottom head, a phenomenon experienced withcommercial reactors at relatively low levels of liquid in the recyclestream.

SUMMARY OF THE INVENTION

It has now been found that by cooling the recycle gas stream to atemperature sufficient to form a liquid and a gas and by separating theliquid from the gas and then feeding the liquid directly to thefluidised bed, the total amount of liquid which may be reintroduced intothe fluidised bed polymerisation reactor for the purpose of cooling thebed by evaporation of the liquid can be increased thereby enhancing thelevel of cooling to achieve higher levels of productivity.

Thus according to the present invention there is provided a continuousgas fluidised bed process for the polymerisation of olefin monomerselected from (a) ethylene, (b) propylene (c) mixtures of ethylene andpropylene and (d) mixtures of a, b or c with one or more otheralpha-olefins in a fluidised bed reactor by continuously recycling agaseous stream comprising at least some of the ethylene and/or propylenethrough a fluidised bed in said reactor in the presence of apolymerisation catalyst under reactive conditions, at least part of thesaid gaseous stream withdrawn from said reactor being cooled to atemperature at which liquid condenses out, separating at least part ofthe condensed liquid from the gaseous stream and introducing at leastpart of the separated liquid directly into the fluidised bed at or abovethe point at which the gaseous stream passing through the fluidised bedhas substantially reached the temperature of the gaseous stream beingwithdrawn from the reactor.

The gaseous recycle stream withdrawn from the reactor (hereafterreferred to as the "unreacted fluids") comprises unreacted gaseousmonomers, and optionally, inert hydrocarbons, reaction activators ormoderators as well as entrained catalyst and polymer particles.

The recycled gaseous stream fed to the reactor additionally comprisessufficient make-up monomers to replace those monomers polymerised in thereactor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show, respectively, the temperature profile within atypical fluidized bed reactor suitable for use in a gas-phasepolymerization of olefins and the location of the thermocouples in thewalls of the reactor for measuring the temperature profile;

FIG. 2 shows a nozzle suitable for use in the polymerization process ofthe present invention;

FIG. 3 shows diagrammatically a gas-phase fluidized bed polymerizationprocess according to the present invention;

FIG. 4 shows diagrammatically an alternative arrangement for thepolymerization process;

FIG. 5 shows yet a further arrangement for the polymerization process;and

FIG. 6 shows diagrammatically an experimental rig for testing theintroduction of fluid into a fluidized bed reaction according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The process according to the present invention is suitable for themanufacture of polyolefins in the gas phase by the polymerisation of oneor more olefins at least one of which is ethylene or propylene.Preferred alpha-olefins for use in the process of the present inventionare those having from 3 to 8 carbon atoms. However, small quantities ofalpha olefins having more than 8 carbon atoms, for example 9 to 18carbon atoms, can be employed if desired. Thus it is possible to producehomopolymers of ethylene or propylene or copolymers of ethylene orpropylene with one or more C₃ -C₈ alpha-olefins. The preferredalpha-olefins are but-l-ene, pent-l-ene, hex-l-ene, 4-methylpent-l-ene,oct-l-ene and butadiene. Examples of higher olefins that can becopolymerised with the primary ethylene or propylene monomer, or aspartial replacement for the C₃ -C₈ monomer are dec-l-ene and ethylidenenorbornene.

When the process is used for the copolymerisation of ethylene orpropylene with alpha-olefins the ethylene or propylene is present as themajor component of the copolymer, and preferably is present in an amountat least 70% of the total monomers.

The process according to the present invention may be used to prepare awide variety of polymer products for example linear low densitypolyethylene (LLDPE) based on copolymers of ethylene with butene,4-methylpent-l-ene or hexene and high density polyethylene (HDPE) whichcan be for example, homopolyethylene or copolymers of ethylene with asmall portion of higher alpha olefin, for example, butene, pent-l-ene,hex-l-ene or 4-methylpent-l-ene.

The liquid which condenses out of the recycle gaseous stream can be acondensable monomer, e.g. butene, hexene, octene used as a comonomer forthe production of LLDPE or may be an inert condensable liquid, e.g.butane, pentane, hexane.

It is important that the liquid should vaporise within the bed under thepolymerisation conditions being employed so that the desired coolingeffect is obtained and to avoid substantial accumulation of liquidwithin the bed. Suitably at least 95, preferably at least 98 weightpercent and most preferably substantially all of the liquid fed to thebed evaporates therein. In the case of liquid comonomers, some of thecomonomer polymerises in the bed, and such polymerisation can be fromthe liquid and the gas phase. As is well known in conventional gas phasepolymerization or copolymerisation processes, a small proportion of themonomer (and comonomer, if any is used) tend to remain associated(absorbed or dissolved) in the product polymer until the polymer issubjected to subsequent degassing. Such associated quantities or evenhigher quantities of absorbed or dissolved monomer/comonomer can readilybe tolerated within the bed provided that these quantities do notadversely affect the fluidisation characteristics of the bed.

The process is particularly suitable for polymerising olefins at apressure of between 0.5 and 6 MPa and at a temperature of between 30° C.and 130° C. For example for LLDPE production the temperature is suitablyin the range 80-90° C. and for HDPE the temperature is typically 85-105°C. depending on the activity of the catalyst used.

The polymerisation reaction may be carried out in the presence of acatalyst system of the Ziegler-Natta type, consisting of a solidcatalyst essentially comprising a compound of a transition metal and ofa cocatalyst comprising an organic compound of a metal (i.e. anorganometallic compound, for example an alkylaluminium compound).High-activity catalyst systems have already been known for a number ofyears and are capable of producing large quantities of polymer in arelatively short time, and thus make it possible to avoid a step ofremoving catalyst residues from the polymer. These high-activitycatalyst systems generally comprise a solid catalyst consistingessentially of atoms of transition metal, of magnesium and of halogen.It is also possible to use a high-activity catalyst consistingessentially of a chromium oxide activated by a heat treatment andassociated with a granular support based on a refractory oxide. Theprocess is also suitable for use with metallocene catalysts and Zieglercatalysts supported on silica.

It is an advantage of the process according to the present inventionthat the improved cooling effect is particularly beneficial forpolymerisation processes using highly active catalysts for examplemetallocene catalysts.

The catalyst may suitably be employed in the form of a prepolymer powderprepared beforehand during a prepolymerization stage with the aid of acatalyst as described above. The prepolymerization may be carried out byany suitable process, for example, polymerisation in a liquidhydrocarbon diluent or in the gas phase using a batch process, asemi-continuous process or a continuous process.

The preferred process according to the present invention is one whereinsubstantially the whole of the recycle gaseous stream is cooled andseparated and wherein substantially the whole of the separated liquid isintroduced into the fluidised bed.

In an alternative embodiment of the present invention the recyclegaseous stream is divided into a first stream and a second stream. Thefirst stream is passed directly to the reactor in a conventional way byinjection below the fluidisation grid and the second stream is cooledand the stream separated into a gas and a liquid stream. The gas streammay be returned to the first stream and reintroduced into the reactorbelow the bed, for example, below the fluidisation grid if such a gridis employed. The separated liquid is introduced into the fluidised bedaccording to the present invention.

The recycle gaseous stream is suitably cooled by means of a heatexchanger or exchangers to a temperature such that liquid is condensedin the gas stream. Suitable heat exchangers are well known in the art.

The gaseous stream leaving the top of the reactor can entrain a quantityof catalyst and polymer particles and these may be removed if desiredfrom the recycle gaseous stream by means of a cyclone. A smallproportion of these particles or fines may remain entrained in therecycle gaseous stream and, after cooling and separating the liquid fromthe gas, the fines can, if desired, be reintroduced into the fluidisedbed together with the separated liquid stream.

The recycle gas stream may also comprise inert hydrocarbons used for theinjection of catalyst, reaction activators or moderators into thereactor.

Make-up monomers for example ethylene to replace monomers consumed bythe polymerisation reaction may be added to the recycle gas stream atany suitable location.

Condensable monomers, for example, butene, hexene, 4-methylpent-l-eneand octene, which can, for example, be used as comonomers for theproduction of LLDPE, or inert condensable liquids, for example, pentane,isopentane, butane and hexane, may be introduced as liquids.

Inert condensable liquids, for example, pentane may for example beinjected into the recycle gaseous stream between the heat exchanger andthe separator. For the preparation of LLDPE, the comonomer, for example,butene can, if desired, be injected into the recycle gaseous streambefore passage to the heat exchanger.

Suitable means for separating the liquid are for example cycloneseparators, large vessels which reduce the velocity of the gas stream toeffect separation (knock-out drums), demister type gas-liquid separatorsand liquid scrubbers, for example, venturi scrubbers. Such separatorsare well known in the art.

The use of a demister type of gas-liquid separator is particularlyadvantageous in the process of the present invention.

The use of a cyclone separator in the recycle gas stream, prior to thegas-liquid separator is preferred. This removes the majority of thefines from the gaseous stream leaving the reactor thereby facilitatingthe use of a demister separator and also reducing the possibility offouling of the separator resulting in a more efficient operation.

A further advantage of using a demister type of separator is that thepressure drop within the separator can be lower than in other types ofseparators thereby enhancing the efficiency of the overall process.

A particularly suitable demister separator for use in the process of thepresent invention is a commercially available vertical gas separatorknown as a "Peerless" (Type DPV P8X). This type of separator uses thecoalescence of liquid droplets on a vane arrangement to separate theliquid from the gas. A large liquid reservoir is provided in the bottomof the separator for collection of the liquid. The liquid reservoirenables the liquid to be stored thereby providing control over thedischarge of the liquid from the separator. This type of separator isvery efficient and gives substantially 100% separation of condensedliquid from the gas stream.

If desired, a filter mesh, or other suitable means, may be arranged inthe liquid reservoir of the separator to collect any remaining finespresent in the separated liquid.

The separated liquid is suitably introduced into the fluidised bed at orabove the point at which the recycle gaseous stream being fed to thereactor has substantially reached the temperature of the recycle gaseousstream being withdrawn from the reactor. The introduction of theseparated liquid may be at a plurality of points within this region ofthe fluidised bed and these may be at different heights within thisregion. The point or points of introduction of the liquid are arrangedsuch that the local concentration of liquid does not adversely affectthe fluidisation of the bed or the quality of the product, and to enablethe liquid to disperse rapidly from each point and vaporise in the bedto remove the heat of polymerisation from the exothermic reaction. Inthis way the amount of liquid introduced for cooling purposes may muchmore closely approach the maximum loading that can be tolerated withoutdisturbing the fluidisation characteristics of the bed and hence offersthe opportunity to achieve enhanced levels of reactor productivity.

The liquid can, if desired, be introduced into the fluidised bed atdifferent heights within the bed. Such a technique can facilitateimproved control over comonomer incorporation. Controlled metering ofliquid into the fluidised bed provides useful additional control overthe temperature profile of the bed and, in the case that the liquidcontains comonomer, provides useful control over the comonomerincorporation into the copolymer.

The liquid is preferably introduced into the lower part of the region ofthe fluidised bed at which the recycle gaseous stream has substantiallyreached the temperature of the gaseous stream being withdrawn from thereactor. Commercial processes for the gas fluidised bed polymerisationof olefins are generally operated under substantially isothermal, steadystate conditions. However, although at least a major portion of thefluidised bed is maintained at the desired substantially isothermalpolymerisation temperature, there normally exists a temperature gradientin the region of the bed immediately above the point of introduction ofthe cooled recycle gaseous stream into the bed. The lower temperaturelimit of this region wherein the temperature gradient exists is thetemperature of the incoming cool recycle gas stream, and the upper limitis the substantially isothermal bed temperature. In commercial reactorsof the type which employ a fluidisation grid, this temperature gradientnormally exists in a layer of about 15 to 30 cm (6 to 12 inches) abovethe grid.

In order to gain the maximum benefit of the cooling of the separatedliquid it is important that the liquid is introduced into the bed abovethe region where this temperature gradient exists, i.e. in the part ofthe bed which has substantially reached the temperature of the gaseousstream leaving the reactor.

The point or points of introduction of the liquid into the fluidised bedmay for example be approximately 50-70 cm above the fluidisation grid.

In practice, the process according to the present invention may becarried out, for example, by first determining the temperature profilewithin the fluidised bed during polymerisation using, for example,thermocouples located in or on the walls of the reactor. The point orpoints of introduction of the liquid is/are then arranged to ensure thatthe liquid enters into the region of the bed at which the recyclegaseous stream has substantially reached the temperature of the gaseousstream being withdrawn from the reactor.

FIG. 1A represents the temperature profile within a typical fluidisedbed reactor suitable for use in the gas-phase polymerisation of olefins.

The temperature profile is shown in a fluidised bed used to prepare HDPEat a rate of 23.7 tonnes/hr. The temperatures were measured usingthermocouples located on the walls of the reactor corresponding todifferent positions (1-5) within the fluidised bed. The locations of 1-5in the fluidised bed reactor are shown in FIG. 1B.

The level of the fluidisation grid (A) and the top of the fluidised bed(B) are indicated on the temperature profile and the diagram. Thetemperature gradient referred to above can be seen as the region betweenposition 1 and position 3. The region in which the recycle gaseousstream has substantially reached the temperature of the unreacted fluidsleaving the reactor is shown as the region between position 3 andposition 5. It is into this region that the separated liquid isintroduced into the fluidised bed in accordance with the processaccording to the present invention.

The liquid is preferably introduced into the fluidised bed in the lowerpart of this region i.e. just above position 3 on the temperatureprofile in FIG. 1A.

By increasing the amount of liquid which may be introduced into thefluidised bed, higher levels of productivity may be achieved due to theincreased cooling capacity. The space time yield can thereby be improvedcompared with other gas phase fluidised bed polymerisation processes.

A further advantage of the process of the present invention is that byintroducing the liquid separately into the fluidised bed, accuratemetering means can be employed to regulate the delivery of the liquid tothe bed. This technique facilitates improved control of the cooling, andprovides improved control over the delivery to the bed of any liquidcomonomer fed in this manner. Thus the process of the present inventioncan be operated in a manner that does not rely, for example, on any needto maintain liquid entrained in the recycle gas stream. Consequently thequantity of liquid fed to the bed can be varied over much broader limitsthan hitherto. The improved control over the rate of addition to the bedof comonomer or inert hydrocarbons may, for example, be used to controlthe density of the polymer formed and the space time yield at which suchpolymer is formed.

It is important to ensure that the temperature within the fluidised bedis maintained at a level which is below the sintering temperature of thepolyolefin constituting the bed.

The gas from the separator is recycled to the bed, normally into thebottom of the reactor. If a fluidisation grid is employed, such recycleis normally to the region below the grid, and the grid facilitatesuniform distribution of the gas to fluidise the bed. The use of afluidisation grid is preferred. Fluidisation grids suitable for use inthe process of the present invention can be of conventional design, forexample, a flat or dished plate perforated by a plurality of holesdistributed more or less uniformly across its surface. The holes may forexample be of diameter of about 5 mm.

The process of the present invention is operated with a gas velocity inthe fluidised bed which must he greater than or equal to that requiredfor fluidisation of the bed. The minimum gas velocity is generallyapproximately 6 cm/sec but the process of the present invention ispreferably carried out using a gas velocity in the range 40 to 100, mostpreferably 50 to 70 cm/sec.

In the process according to the present invention the catalyst orprepolymer can, if desired, be introduced into the fluidised beddirectly with the separated liquid stream. This technique can lead toimproved dispersion of the catalyst or prepolymer in the bed.

If desired, liquid or liquid-soluble additives, for example, activators,cocatalysts and the like, can be introduced into the bed together withthe condensed liquid by the process according to the present invention.

In the case that the process of the present invention is employed tomake ethylene homo- or copolymers, make-up ethylene, for example, toreplace the ethylene consumed during the polymerisation, may beadvantageously introduced into the separated gas stream prior to itsreintroduction into the bed (for example below the fluidisation grid ifsuch is employed). By adding the make-up ethylene to the separated gasstream rather than into the recycle gaseous stream before separation,the quantity of liquid which may be recovered from the separator may beincreased and the productivity improved.

The separated liquid stream may be subjected to additional cooling (e.g.using refrigeration techniques) before being introduced into thefluidised bed. This allows an even greater cooling effect in the bedthan is provided by the liquid evaporative effect (latent heat ofevaporation) alone, thereby providing further potential increases inproductivity of the process. Cooling of the separated liquid stream maybe achieved by use of suitable cooling means e.g. a simple heatexchanger or refrigerator located between the separator and the reactor.A further advantage of this particular aspect of the present inventionis that, by cooling the liquid before introduction into the fluidisedbed, any tendency for catalyst or prepolymer which may be contained inthe liquid stream to cause polymerisation before introduction into thebed will be reduced.

The liquid may be introduced into the fluidised-bed by suitably arrangedinjection means. A single injection means may be used or a plurality ofinjection means may be arranged within the fluidised bed.

A preferred arrangement is to provide a plurality of injection meanssubstantially equally spaced in the fluidised bed in the region of theintroduction of the liquid. The number of injection means used is thatnumber which is required to provide sufficient penetration anddispersion of liquid at each injection means to achieve good dispersionof liquid across the bed. A preferred number of injection means is four.

Each of the injection means may, if desired, be supplied with theseparated liquid by means of a common conduit suitably arranged withinthe reactor. This can be provided, for example, by means of a conduitpassing up through the centre of the reactor.

The injection means are preferably arranged such that they protrudesubstantially vertically into the fluidised bed, but may be arrangedsuch that they protrude from the walls of the reactor in a substantiallyhorizontal direction.

The rate at which the liquid can be introduced into the bed dependsprimarily on the degree of cooling desired in the bed, and this in turndepends on the desired rate of production from the bed. The rates ofproduction obtainable from commercial fluidised bed polymerisationprocesses for the polymerisation of olefins depend, inter alia on theactivity of the catalysts employed, and on the kinetics of suchcatalysts. Thus for example, when catalysts having very high activityare employed, and high production rates are desired, the rate of liquidaddition will be high. Typical rates of liquid introduction may be, forexample, in the range 0.3 to 4.9 cubic meters of liquid per cubic meterof bed material per hour, or even higher. For conventional Zieglercatalysts of the "superactive" type (i.e. those based on transitionmetal, magnesium halide and organometallic cocatalyst,) the rate ofliquid addition may be, for example, in the range 0.5 to 1.5 cubicmeters of liquid per cubic meter of bed material per hour.

In the process of the present invention the weight ratio of liquid:totalgas which may be introduced into the bed can be for example in the range1:100 to 2:1, preferably in the range 5:100 to 85:100, most preferablyin the range 6:100 to 25:100. By total gas is meant the gas which isreturned to the reactor to fluidise the bed together with any gas usedto assist in the operation of the injection means, e.g. atomising gas.

By injecting the liquid into the fluidised bed in this way any catalystwhich is present in the liquid may benefit from the localised coolingeffect of the liquid penetration surrounding each injection means whichmay avoid hot spots and consequent agglomeration.

Any other suitable injection means may be used provided the penetrationand dispersion of the liquid into the bed from such means is sufficientto achieve a good dispersion of liquid across the bed.

The preferred injection means is a nozzle or a plurality of nozzleswhich include gas-induced atomising nozzles in which a gas is used toassist in the injection of the liquid, or liquid-only spray-typenozzles.

According to another aspect of the present invention there is provided acontinuous gas fluidised bed process for the polymerisation of olefinmonomer selected from (a) ethylene, (b) propylene (c) mixtures ofethylene and propylene and (d) mixtures of a, b or c with one or moreother alpha-olefins in a fluidised bed reactor by continuously recyclinga gaseous stream comprising at least some of the ethylene and/orpropylene through a fluidised bed in said reactor in the presence of apolymerisation catalyst under reactive conditions, at least part of thesaid gaseous stream withdrawn from said reactor being cooled to atemperature at which liquid condenses out, separating at least part ofthe condensed liquid from the gaseous stream and introducing the liquiddirectly into the fluidised bed by one or more liquid-only nozzles orgas-induced atomising nozzles. The fluidised bed is preferably supportedabove a fluidisation grid.

The injection means are suitably nozzles which protrude into the bedthrough the reactor wall (or through a supporting grid for the bed) andwhich carry one or more jet outlets to deliver the liquid to the bed.

It is important in the process of the present invention to achieve gooddispersion and penetration of the liquid in the bed. Factors which areimportant in achieving good penetration and dispersion are the momentumand direction of the liquid entering the bed, the number of points ofintroduction of the liquid per unit crossectional area of the bed, andthe spatial arrangement of the points of introduction of the liquid.

A further aspect of the present invention provides a process for thepolymerisation of olefin monomer, the olefin monomer preferably beingselected from (a) ethylene, (b) propylene (c) mixtures of ethylene andpropylene and (d) mixtures of a, b or c with one or more otheralpha-olefin olefins, in a fluidised bed reactor by continuouslyrecycling a gaseous stream comprising the monomer, preferably comprisingat least the ethylene and/or propylene, through a fluidised bed in saidreactor in the presence of a polymerisation catalyst under reactiveconditions, at least part of the said gaseous stream withdrawn from saidreactor being cooled to a temperature at which liquid condenses out,separating at least part of the condensed liquid from the gaseous streamand introducing at least part of the separated liquid directly into thefluidised bed at or above the point at which the gaseous stream passingthrough the fluidised bed has substantially reached the temperature ofthe gaseous stream being withdrawn from the reactor, said liquid beingintroduced into said reactor as one or more jets of liquid alone, or oneor more jets of liquid and gas, from one or more jet outlets, each jethaving a horizontal momentum flux in the case of the liquid only jets ofat least 100×10³ Kg s⁻¹ m⁻² ×m s⁻¹ and in the gas/liquid jets of 200 Kgs⁻¹ m⁻² ×m s⁻¹ wherein the horizontal momentum flux is defined as themass flow rate of liquid (kilogrammes per second) in the horizontaldirection per unit crossectional area (square meters) of the jet outletfrom which it emerges, multiplied by the horizontal component of thevelocity (meters per second) of the jet.

Preferably the momentum flux of each of the liquid or liquid/gas jets isat least 250×10³ and most preferably at least 300×10³ Kg s⁻¹ m⁻² ×m s⁻¹.Particularly preferred is the use of a horizontal momentum flux in therange 300×10³ to 500×10³ Kg s⁻¹ m⁻² ×m s⁻¹. In the case that the liquidjet emerges from the jet outlet in a direction other than horizontal,the horizontal component of the velocity of the jet is calculated fromCosine Q°×actual jet velocity, wherein Q° is the angle the jet makeswith the horizontal.

The direction of motion of the one or more liquid or liquid/gas jetsinto the bed is preferably substantially horizontal. In the case thatone or more of the jet outlets deliver the liquid or liquid/gas jets ina direction other than horizontal, preferably these are directed at anangle not greater than 45°, most preferably not more than 20° to thehorizontal.

The one or more nozzles are suitably each equipped with one or more jetoutlets. The number of nozzles, and the number and distribution of thejet outlets are important factors in obtaining good distribution ofliquid within the bed. If a plurality of nozzles are employed, they arepreferably vertically disposed and spaced horizontally and substantiallyequidistant from one another. In this case, they are also preferablyspaced equidistant from one another and from the vertical wall of thefluidised bed. The number of nozzles per 10 square meters of thehorizontal crossectional area of the bed is preferably in the range 1 to4, most preferably in the range 2 to 3. Where the calculated number isnot an integer, it is preferably rounded up to an integer. The number ofjet outlets in each nozzle is preferably in the range 1 to 40 mostpreferably in the range 3 to 16. In the case that the nozzle containsmore than one jet outlet, the jet outlets are preferably arrangedcircumferentially and equidistant from one another around the nozzle.

As indicated above, the jets of liquid may consist solely of liquid ormay comprise a liquid/gas mixture. Such gas may be merely carried in theliquid, or may be employed to atomise the liquid, or to provide motiveforce to propel the liquid.

A suitable gas-induced atomising nozzle for use in the process accordingto the present invention comprises

(a) at least one inlet for a pressurised liquid,

(b) at least one inlet for an atomising gas,

(c) a mixing chamber to mix said liquid and gas, and

(d) at least one outlet through which said mixture is discharged.

The atomising gas may suitably be an inert gas for example nitrogen butis preferably make-up ethylene.

Each nozzle may be provided with a plurality of outlets of suitableconfiguration. The outlets may for example comprise circular holes,slots, ellipsoids or other suitable configurations. Each nozzle maycomprise a plurality of outlets of varying configuration.

The size of the outlets is preferably such that there is little pressuredrop through the outlets.

The outlets are preferably symmetrically arranged around thecircumference of each nozzle but may also be arranged asymmetricallytherein.

The atomising gas supply to each nozzle is maintained at a pressuresufficient to break the liquid into small droplets and to preventparticle ingress from the fluidised bed or particle blockage of theoutlets of the nozzle.

The relative size of the mixing chamber is arranged to ensure optimumatomisation. The volume of the mixing (atomising) chamber relative tothe volume of liquid passing through the chamber expressed as: Volume ofmixing chamber (in cubic cm)/Liquid flowrate (cubic cm per second), ispreferably in the range 5×10⁻³ to 5×10⁻¹ seconds.

The velocity of the liquid is preferably maintained at a velocitysufficient to ensure that any particles, for example fines, do notseparate out of the liquid stream.

The weight ratio of atomising gas to liquid supplied to each nozzle istypically in the range 5:95 to 25:75.

FIG. 2 represents a nozzle suitable for use in the process according tothe present invention.

In the Figure the nozzle comprises a housing 7 comprising an upperregion 8 and a lower region 9. The upper region is provided with anumber of outlets 10 arranged on its circumference and a mixing chamber11 arranged therein. The lower region is provided with a centrallylocated conduit 12 opening into the mixing chamber and an outer conduit13 located around the inner conduit. The conduit 13 communicates withthe mixing chamber by suitably arranged openings 14. Pressurised liquidis supplied to the nozzle by conduit 13 and atomising gas is supplied toconduit 12. The lower region of the nozzle 9 is connected byconventional means to a supply of pressurised liquid and atomising gas.After mixing with the gas in the chamber 11 the liquid is dischargedfrom the nozzle via the outlets 10 as an atomised spray.

A preferred gas-induced atomiser nozzle is one wherein the outletscomprise a series of substantially horizontal slots arranged around thecircumference of the nozzle. The nozzle may also comprise a verticallyorientated hole or holes located to ensure that any particles adheringto the top of the nozzle may be removed by the pressurised gas-liquidmixture.

The slots may typically be of a size equivalent to a hole of diameter ofabout 6.5 mm and may for example be of dimension 0.75 mm ×3.5 mm.

The injection means may alternatively comprise liquid-only spray-typenozzle or nozzles.

A suitable liquid-only spray nozzle for use in the process according tothe present invention comprises at least one inlet for pressurisedliquid and at least one outlet for said pressurised liquid, sufficientliquid pressure being maintained within the nozzle to ensure that theliquid emerging from the outlet has the desired momentum flux.

The pressure drop in each nozzle can be regulated if desired, forexample, by the use of restrictive devices such as valves.

The outlets may comprise similar configurations as defined above for thegas-induced atomiser nozzles. The preferred configuration for theoutlets in the liquid spray nozzle is circular holes. The holes arepreferably of diameter in the range 0.5 to 5 mm., most preferably in therange 0.5 to 2.5 mm.

The droplet size of the liquid is influenced by a number of factors inparticular in the gas-induced atomiser nozzles by the ratio of theliquid to atomising gas supplied to the nozzle and the size andconfiguration of the atomising chamber. A desirable liquid droplet sizefor a gas-induced atomiser nozzle is from about 50 micron to about 1000microns. In the case of the liquid spray nozzles the liquid droplet sizeis influenced primarily by the pressure drop in the nozzle and the sizeand configuration of the outlets. A desired liquid droplet size for theliquid spray nozzle is from about 2000 microns to about 4000 microns.Such droplets can be generated, for example, by disruption of a liquidjet by the motion of the solid particles forming the bed.

The pressure drop in either type of nozzle must be sufficient to preventthe ingress of particles from the fluidised bed. In the gas-inducedatomiser nozzle the pressure drop is suitably in the range 2 to 7,preferably 3 to 5 bar and, in the liquid spray nozzles in the range 2 to7, preferably 4 to 5 bar.

In the event of a failure in the supply of liquid and/or atomising gasto either of the nozzles suitable means are arranged to provide for anemergency gas purge to prevent blockage of the nozzle by the ingress ofparticles from the fluidised bed. A suitable purging gas is nitrogen.

It is important that the outlets of the gas-induced atomising nozzles orthe liquid-only nozzles are of sufficient size to allow for the passageof any fines which may be present in the separated liquid stream.

In either type of nozzle the outlets may be arranged at different levelswithin each nozzle. For example the outlets may be arranged in a seriesof rows.

In the type of nozzle illustrated in FIG. 2, the number of outlets oneach nozzle is preferably between 4 and 40, for example between 20 and40, most preferably from 4 to 16. The diameter of such a nozzle ispreferably in the range 4 to 30 cm, e.g. 10 to 30 cm, and is mostpreferably of approximately 7 to 12 cm.

Other types of nozzle may also be suitable for use in the process of thepresent invention for example ultrasonic nozzles.

Before commencing the introduction of liquid by use of the processaccording to the present invention the gas phase fluidised bedpolymerisation may be started in a conventional manner for example bycharging the bed with particulate polymer particles and then initiatingthe gas flow through the bed.

When appropriate the liquid may be introduced into the bed, for exampleusing the injection means described above. During start-up the atomisinggas supply to the gas-induced nozzles or the purging gas flow to theliquid spray nozzles must be maintained at a velocity sufficient toprevent the ingress of particles into the outlets of the nozzles.

Processes according to the present invention will now be illustratedwith reference to the accompanying drawings.

FIGS. 3-5 show diagrammatically processes according to the presentinvention.

FIG. 3 illustrates a gas-phase fluidised bed reactor consistingessentially of a reactor body 15 which is generally an upright cylinderhaving a fluidisation grid 16 located in its base. The reactor bodycomprises a fluidised bed 17 and a velocity reduction zone 18 which isgenerally of increased cross-section compared to the fluidised bed.

The gaseous reaction mixture leaving the top of the fluidised bedreactor constitutes recycle gaseous stream and is passed via line 19 toa cyclone 20 for the separation of the majority of the fines. Removedfines may suitably be returned to the fluidised bed. The recycle gaseousstream leaving the cyclone passes to a first heat exchanger 21 and acompressor 22. A second heat exchanger 23 is present to remove the heatof compression after the recycle gaseous stream has passed through thecompressor 22.

The heat exchanger or exchangers can be arranged either upstream ordownstream.of the compressor 22.

After cooling and compression to a temperature such that a condensate isformed, the resultant gas-liquid mixture is passed to the separator 24where the liquid is removed.

The gas leaving the separator is recycled via line 25 to the bottom ofthe reactor 15. The gas is passed via the fluidisation grid 16 to thebed thereby ensuring that the bed is maintained in a fluidisedcondition.

The separated liquid from the separator 24 is passed via line 25' to thereactor 15. If necessary a pump 26 may be suitably located in line 25'.

Catalyst or prepolymer are fed to the reactor via line 27 into theseparated liquid stream.

Product polymer particles may be suitably removed from the reactor vialine 28.

The arrangement shown in FIG. 3 is particularly suitable for use whenretrofitting existing gas phase polymerisation reactors using fluidisedbed processes.

FIG. 4 illustrates an alternative arrangement for performing the processof the present invention. In this arrangement the compressor 22 islocated in line 25 after separation of the recycle gaseous stream by theseparator 24. This has the advantage that the compressor has a reducedquantity of gas to compress and can therefore be of reduced sizeachieving a better process optimisation and cost.

FIG. 5 illustrates a further arrangement for performing the process ofthe present invention whereby the compressor 22 is again arranged inline 25 after the separator 24 but before the second heat exchanger 23which is located in the separated gas stream rather than located beforethe separator. Again this arrangement gives a better processoptimisation.

The process according to the present invention will now be furtherillustrated with reference to the following Examples.

EXAMPLES 1 To 11

Computer generated Examples were obtained for the simulatedpolymerisation of olefins in a gas-phase fluidised bed reactor underconditions according to the present invention (Examples 1 to 5, 9 and10) and for comparison under conventional conditions with no separatedliquid in the recycle stream (Examples 6 to 8 and 11).

Examples 1 to 8 represent copolymerisations of ethylene with a varietyof alpha-olefins using a conventional Ziegler catalyst and Examples 9 to11 represent homopolymerisation of ethylene using a conventional silicasupported chromium oxide catalyst.

The space time yield and the reactor inlet temperature were computedusing a heat balance computer program with an accuracy of ±15%. The dewpoint temperature and the rate of flow of the recycled liquid werecomputed for the polymerisation system using a conventional softwareprogram with an accuracy of about ±10%.

Examples 1, 3, 4 and 10 most closely represent typical processconditions for performing the process according to the presentinvention.

The results are given in Table 1 and Table 2 and clearly show improvedspace time yields obtainable using the process of the present invention.

The "% Liquid in Recycled Stream" in Tables 1 and 2 represents, as apercentage, the total weight of liquid recycled through the injectionmeans divided by the total weight of gas (recycle gas plus any atomisinggas).

EXAMPLES 12 To 15

An experimental rig was used to test the introduction of liquid into afluidised bed by use of injection means as described above. Thearrangement of the test rig is shown in FIG. 6. The test rig comprisesan aluminium fluidisation vessel 50 having a velocity reduction zone 56containing a bed 51 of polyethylene powder (high density or linear lowdensity polyethylene), previously prepared by gas fluidised bedpolymerisation of ethylene in an industrial scale gas fluidised bedplant. The bed 51 was fluidised by passing a continuous stream of drynitrogen gas through line 52 and preheater 53 into the base chamber 54of vessel 50, and thence into the bed through grid 55. The nitrogen gaswas supplied from a commercial liquid nitrogen supply tank, and thequantity of nitrogen supplied to fluidise the bed and the gas pressurein the svstem were controlled by means of valves 57 and 69, the volumeflow rate being determined using a conventional turbine meter (notshown). The preheater unit had a nominal heating capacity of 72 kW whichwas controllable to heat the nitrogen gas to the desired temperature.Volatile liquid hydrocarbon 58 (1-hexene or n-pentane) was introducedinto the fluidised bed 51 from a cooler/demister tank 59 by means of apump 60 and line 61. The volatile liquid hydrocarbon entered the bedthrough nozzle/jet outlet arrangement 62 which penetrated into the bed.Various nozzle/jet outlet arrangements were tested, some being of theliquid-only type, and others being of the gas atomising type. For thelatter type, atomising gas was introduced through line 63 (for anexample of this type of nozzle, see FIG. 2 of the accompanyingdrawings). Volatile liquid hydrocarbon entering the fluidised bedthrough the nozzle/jet outlet arrangement 62 evaporated in the bed thuscausing cooling by absorbing latent heat of evaporation. The nitrogenfluidising gas and the accompanying volatilised liquid hydrocarbonemerged from the top of the bed into the velocity reduction zone 56wherein the bulk of any polyethylene powder entrained in the gas streamfell back into the bed. The gas then passed into line 64, filter unit 65and through non-return valve 66 into the cooler/demister tank 59. Thecooler/demister tank 59 contained two heat exchangers 67, 68. Heatexchanger 67 was cooled by passage therethrough of cold water, and 68was cooled by circulating a refrigerated ethylene glycol/waterantifreeze solution. Passage of the gas over the heat exchangers 67, 68cooled the gas and caused the liquid hydrocarbon (hexene or pentane) tocondense out. The condensed hydrocarbon collected in the base of tank59, from whence it was recycled back to the bed. The nitrogen gas thussubstantially freed from hydrocarbon was then passed throughback-pressure regulating valve 69 to the atmosphere. The fluidisationand vaporisation of the liquid hydrocarbon in the bed was monitoredusing commercially available X-ray imaging apparatus comprising an X-raysource 70, an image intensifier 71 and a CCD (charge coupled device)video camera 72 the output of which was continuously recorded on a videotape recorder (not shown). The X-ray source, image intensifier and videocamera were mounted on a moveable gantry 73 to enable the field of viewof the bed to be changed at will.

The process of the present invention provides substantial improvementsin productivity of gas fluidised bed polymerisation processes overexisting processes. The process of the present invention can be employedin new plant or can be employed in existing plant to obtain substantialincreases in productivity and better control of liquid addition to thebed. In the case of the installation of new plant, substantialreductions in capital costs can be achieved by using smaller reactionvessels, compressors and other ancillary equipment than would have beennecessary to achieve comparable productivities from conventional plant.In the case of existing plant, modification of such plant in accordancewith the present invention provides substantial increases inproductivity and improved control of the process.

The results of the tests are shown in Table 3 wherein Examples 12, 14and 15 are in accordance with the present invention, and Example 13 isby way of comparison. Example 12 and Comparative Example 13 illustratethe use of the same nozzle arrangement, but in the Comparative Example,the addition of liquid to the "cold" zone of a gas fluidised bedpolymerisation is simulated by running the bed at 45° C. in comparisonwith the 98° C. employed in Example 12. Under these circumstances lumpsof polymer wet with liquid hydrocarbon formed around the nozzle.Examples 12, 14 and comparative Example 13 employed gas inducedatomising nozzles, and Example 15 a liquid only nozzle. Examples 12, 14and 15 all produced good penetration and dispersion of the liquidhydrocarbon, the liquid penetration being stopped only by the vesselwall. In Comparative Example 13, liquid penetration was inhibited by theformation of agglomerated lumps of polymer/liquid hydrocarbon

                                      TABLE 1                                     __________________________________________________________________________                 Example                                                                       1     2     3     4     5     6                                               Product                                                                       C2/C4 C2/C4 C2/C6 C2/C6 C2/C4 C2/C4                                Copolymer Copolymer Copolymer Copolymer Copolymer Copolymer                 __________________________________________________________________________    Reactor Pressure (bar)                                                                     24    26    24    24    26    24                                   Reactor Temperature (° C.)  93      93      82      82  76                                                      93                                   Process Gas (% mol)                                                           Ethylene         37.5    42.3    29.1    31.2    55.8    28.5                 Ethane          14.8   18.5    15.9    14.2      3.1   25.2                   Hydrogen         26.2    29.6     4.9     10     11.1    19.9                 Nitrogen          9.3     6      39.1     35  5.7   24.3                      Butene            0.2     8.2         18.6    8.14                            Pentane          10.4     10   1  1      5.4                                  Hexene                     5.3                                                4-MPI               7.7                                                       Others egoligomers      1.6    1.4    2.3     3.3      0.3   1.96                                                       Gas Velocity (cm/sec)   60                                                     60     60      60  60     60       Bed Height (metres)     14.5    14.5    14.5    14.5    14.5    14.5                                                    Space Time Yield (Kg/m.sup.3                                                 h)     140     193     105                                                    116     193      74                  Reactor Inlet Temperature     46.5    36.2    53.4    48.8    44.8                                                     49.9                                 (° C.)                                                                 Dew Point Temperature (° C.)      70.9    78.9    67.7    69.5                                                   61.6    46.1                        % Liquid in Recycled Stream       14.4    21.5    11    11.3   21.3         __________________________________________________________________________                                               0                              

                                      TABLE 2                                     __________________________________________________________________________                 Example                                                                       7     8     9      10     11                                                  Product                                                                       C2/C4 C2/C4 C2     C2     C2                                       Copolymer Copolymer Homopolymer Homopolymer Homopolymer                     __________________________________________________________________________    Reactor Pressure (bar)                                                                     24    24    24     24     20                                       Reactor Temperature (° C.)     74     76      103  103  103                                                  Process Gas (% mol)                     Ethylene                  36.7  37.5     29.1      29.1      35                                                     Ethane                 7.4   9.7                                                 16.4     17        17.5                                                    Hydrogen        8.4   7.5                                                    12.5      12.5      15                   Nitrogen        24.3  31.9     24.4      25.6      28.1                       Butene          19.3  12.15                                                   Pentane             13.3      11.2                                            Hexene                                                                        4-MP1                                                                         Others egoligomers      3.9   1.25     4.3  4.6  4.4                          Gas Velocity (cm/sec)    60     60      60    60  60                          Bed Height (meters)      14.5  14.5     14.5      14.5      14.5                                                    Space Time Yield (Kg/m.sup.3 h)                                                  55     55      193  178  75                                                Reactor Inlet Temperature                                                    50.1  49.7     36.4      36.1                                                 57                                       (° C.)                                                                 Dew Point Temperature (° C.)       38.2  23.5     62.3      56.3                                             -44.5                                   % Liquid in Recycled Stream   0      0  21   15.7     0                     __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                               EXAMPLE                                                                         12        13        14      15                                       ______________________________________                                        Nozzle type                                                                            Gas       Gas       Gas     Liquid-only                                 atomising atomising atomising                                                Outlet type 4 horizontal   4 horizontal   4 horizontal  2 holes of                                                            slots           slots                                             slots   1.75 mm                                               diam.                                                     Location above      52  52   10      52                                       grid (cm)                                                                     Fluidising gas      45  42   52      38                                       velocity (cm/s)                                                               Bed temp. ° C.      98  45   78      97                                Pressure (MPa)       1.01  0.97   0.78 0.75                                   Bed material         HDPE  HDPE LLDPE BP HDPE BP                                                       BP GRADE BP Grade Grade 0209 Grade 6070                                                     6070  6070                             Bed charge  60  58.5 61.2 58.0                                                (Kg)                                                                          Liquid  hexene hexene  n-pentane hexene                                       Liquid flow 1.65  1.48 1.78 0.69                                              (M.sup.3 /h)                                                                  Liquid pressure   0.33  0.32 0.38 0.54                                        to nozzle                                                                     (MPa)                                                                         N.sub.2 atomising   0.42  0.40 0.45 none                                      gas pressure                                                                  (MPa)                                                                         Atomising gas:  5.4  5.3  5.6 none                                            liquid (mass %)                                                               M.sup.3 liquid per 11.38 10.61   12.80 4.95                                   M.sup.3 bed                                                                   per hour                                                                      Horizontal greater less than 15 greater than greater                          penetration of   than 21   21 than 21                                         liquid (cm)                                                                   % Condensed 105.5 94.6  121.2 46.6                                            liquid (% total                                                               liquid/total                                                                  gas)                                                                        ______________________________________                                    

We claim:
 1. A continuous gas fluidized bed process for thepolymerization of olefin monomer selected from the group consisting of(a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene, and(d) mixtures of (a), (b), or (c) with one or more other alpha-olefinscomprising passing a gaseous stream containing said olefin monomerthrough a fluidized bed in a reactor in the presence of a polymerizationcatalyst under reactive conditions to polymerize some of said monomer,withdrawing a gaseous stream comprising at least some unreacted monomerfrom said reactor; cooling at least part of said gaseous streamwithdrawn from said reactor to a temperature at which liquid condensesout of the gaseous stream, separating at least part of the condensedliquid from the cooled gaseous stream, continuously recycling at leastpart of the separated cooled gas back to the reactor, and introducing atleast part of the separated liquid directly into the fluidized bed at orabove the point at which the gaseous stream passing through thefluidized bed has substantially reached the temperature of the gaseousstream being withdrawn from the reactor, wherein the polymerizationcatalyst is introduced into the fluidized bed directly with theseparated liquid.
 2. The process of claim 1, wherein the polymerizationcatalyst is a metallocene.
 3. The process of claim 1, wherein thepolymerization catalyst is in the form of a prepolymer.
 4. The processof claim 1, wherein liquid or liquid-soluble additives are alsointroduced into the fluidized bed directly with the separated liquid. 5.The process of claim 4, wherein the additives are activators orcocatalysts.
 6. The process of claim 5, wherein the cocatalyst is anorganometallic compound.